1. Field of the Invention
This invention relates generally to hydraulic dampers, and more particularly to a new and improved semi-active damper with an externally mounted valve assembly for selectively varying stiffness of the damper in compression and separately selectively varying stiffness of the damper in rebound.
2. Description of Related Art
Dampers are used in conjunction with automotive suspension systems to absorb unwanted vibrations which occur while driving a vehicle. In order to absorb unwanted vibrations, dampers are generally connected between the body and the suspension of an automobile. A piston is located within the damper which is connected to the body of the automobile through a piston rod. Furthermore, the damper body is connected to the suspension of the automobile. Because the piston is able to limit the flow of damping fluid within the working chamber of the damper as the damper is compressed extended, the damper is able to produce a damping force which counteracts suspension system vibration which wold otherwise be transmitted from the suspension to the body. By further restricting the flow of damping fluid within the working chamber of a damper, greater damping forces are generated by the damper.
In determining the optimal amount of damping that a damper should provide, three vehicle performance characteristics are often considered: ride comfort, vehicle handling and road holding ability. Ride comfort is typically a function of the spring constant of the vehicle's main springs, as well as the spring constant of the occupant's seat, the vehicle's tires the suspension geometry, and the damper. Vehicle handling is related to changes in the vehicle's attitude (i.e., pitch, yaw, and roll). To achieve optimum vehicle handling, relatively large damping forces are required to avoid excessively rapid variation in the vehicle's attitude during acceleration, deceleration, and cornering. Road holding ability is generally dependent on the amount of contact between the vehicle tires and the ground. In order to optimize a vehicle's road holding ability, large damping forces are required as a vehicle passes over irregular surfaces in order to prevent loss of contact between the wheels and ground for an excessive period of time.
Because different driving characteristics require differing amounts of damping force from the damper in order to optimize its performance, it is often desirable to have a damper which can be adjusted to increase or decrease the requisite damping forces. One method for selectively changing a damper's damping characteristics is described in U.S. Pat. No. 4,890,858. This reference discloses a rotary valve for use in controlling a damper. The damper comprises a first valve member which is disposed within the pressure cylinder for establishing a plurality of flow passages. Furthermore, the damper comprises a second valve member also disposed within the pressure cylinder for establishing a second plurality of flow passages. In addition, the damper includes an actuator for providing an accelerating and decelerating force to the second valve member. Finally, control means for controlling displacement of the second valve member are also disclosed.
Because dampers which provide adjustable damping generally use a single valve to control the flow of damping fluid during both compression and rebound, a sensor is generally required to determine whether the damper is in compression or rebound. Not only does this provide a degree of difficultly in terms of sensor placement, there are also disadvantages with respect to the electronics which are required to generate an output indicative of whether the damper is in compression or rebound from the output of the sensor. Accordingly, these systems tend to be somewhat expensive.